This invention relates to a process for separating acid from salt solutions containing acid. More particularly, the invention is directed to a new use for an apparatus comprising bipolar membranes and anion membranes to separate acids from solutions comprising acids and salts.
Electrodialytic water-splitting in a two-compartment cell is well known For example, U.S. Pat. No. 4,391,680 discloses the generation of strongly acidified sodium chloride and aqueous sodium hydroxide by two compartment water-splitting of aqueous sodium chloride. Three compartment electrodialytic water splitters are known in the art. They are disclosed to be comprised of alternating bipolar, anion and cation exchange membranes thereby forming alternating acid, salt and base compartments. U.S. Ser. No. 135,562 discloses three compartment electrodialytic water splitters. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three compartment water splitter to separate the acid from the salt.
U.S. Pat. No. 4,608,141 discloses a multi chamber two compartment electrodialytic water splitter and a method for using the same for the basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 discloses a multi chamber two compartment electrodialytic water splitter and a method for using the same for the acidification of aqueous soluble salts. These two patents review the use of two compartment electrodialytic water splitters and their use to treat salts.
The staging of two conventional two compartment electrodialytic water splitters whereby the base solution is withdrawn from the base compartment of one two compartment water splitter and is fed through the base compartment of the second two compartment water splitter is known. In an attempt to increase the efficiency of bipolar membranes, U.S. Pat. No. 3,111,472 (Oda, et al.) discloses disposing a microporous water permeable cation or neutral membrane in the acid and/or base compartments of the three compartment electrodialytic water splitter.
Although bipolar membranes have been known to be useful for the process of electrodialytic water splitting to generate an acid and a base for many years (Oda et al. U.S. Pat. No. 2,829,095, Chlanda et al. U.S. Pat. No. 3,787,304, Jenczewski et al. U.S. Pat. No. 4,552,635) and their use in various cell configurations has been reported (Oda et al. Japan 2023 ('58) reported in Chemical Abstracts 53:11070b. U.S. Pat. No. 4,536,269 and U.S. Pat. No. 4,608,141) their use as highly selective membranes which effectively transport only H+ or OH- ions therefore making them effective for the purification of acids without the generation of the opposite product has not been realized.
None of the above references disclose separating a salt solution containing an acid using a bipolar membrane, in an apparatus such as a two compartment water splitter, followed by removing the salt solution.
The art, such as U.S. Pat. No. 4,536,269, teaches that cation membranes preferentially permit the selective transport of hydrogen ions over other cations such as metal ions over hydrogen ions, but the selectivity is small.